Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Recently, use of secondary batteries is realized as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. Accordingly, a great deal of research is focused on secondary batteries satisfying various requirements and, in particular, use of lithium secondary batteries with high energy density, high discharge voltage and superior power stability is increasing.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used under severe conditions for 10 years or longer in which charge and discharge at a high current are repeated for a short time, thus requiring considerably superior safety and long lifespan, as compared to conventional small lithium secondary batteries.
In addition, recently, a great deal of research is focused on use of lithium secondary batteries for power storage devices in which unusable power is converted into physical or chemical energy, stored and used as electric energy, as necessary.
Lithium secondary batteries used for large-sized power storage devices should have high energy density and efficiency, long lifespan and, in particular, should secure safety and reliability, since combustion or explosion upon malfunction of systems may cause major accidents.
In this regard, conventional lithium secondary batteries generally utilize a lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such lithium cobalt composite oxide is unsuitable for electric vehicles or large-capacity power storage devices in terms of extreme expensiveness of cobalt as a main component and safety.
Also, in case of lithium manganese composite oxides, manganese is eluted into an electrolyte during charge and discharge at high temperatures and high currents, causing deterioration in battery characteristics. Also, lithium manganese composite oxides disadvantageously have smaller capacity per unit weight than conventional lithium cobalt composite oxides or lithium nickel composite oxides, thus having a limitation in increase in capacity per unit weight. Accordingly, a great deal of research on active materials comprising other elements is underway.
For example, Korean Patent Laid-open No. 2004-0092245 discloses a cathode active material having a spinel structure for 5V lithium secondary batteries, obtained by preparing a spherical precursor powder which comprises compound represented by Li1+x[Ni(1/2+a)Mn(3/2-2a)MOa]O4 (0≦x≦0.1, 0≦a≦0.1) and has a particle size of 1 to 5 μm, and calcining the same at 700° C. to 1,100° C. Also, Korean Patent Laid-open No. 2010-0032395 discloses a surface-modified lithium-containing composite oxide with a perovskite structure for cathode active materials for lithium ion secondary batteries, wherein the lithium-containing composite oxide is represented by LipNxMyOzFa (wherein N is at least one element selected from the group consisting of Co, Mn and Ni, M is at least one element selected from the group consisting of transition metal elements other than N; Al, Sn and alkaline earth metal elements, and 0.9≦p≦1.3, 0.9≦x≦2.0, 0≦y≦0.1, 1.9≦z≦4.2, 0≦a≦0.05). However, these materials still disadvantageously have a limitation in securing structural stability due to repeated deintercalation and intercalation of lithium ions (Li+).
Further, in spite of continuous research to secure stability of cathode active materials, cathode active materials satisfying reliable safety of middle- and large-sized lithium secondary batteries and configurations of lithium secondary batteries comprising the same have not been suggested yet.